The production of cycloalkylaromatic compounds, such as cyclohexylbenzene, is a commercially important reaction since these compounds have utility as chemical feedstocks, solvents and industrial fluids.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process involving alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, the cost of propylene is likely to increase, due to a developing shortage of propylene. Thus, a process that does not require propylene as a feed and coproduces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenol.
One such process involves the catalytic hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts.
An example of such a process is described in, for example, U.S. Pat. No. 3,760,017, which discloses a method for the catalytic hydroalkylation of an aromatic hydrocarbon, such as benzene, to cyclohexylbenzene using a dual function catalyst followed by the conversion of the cyclohexylbenzene to cyclohexanone and phenol by air oxidation and acid decomposition. The dual function catalyst comprises a Group VIII metal selected from the group consisting of cobalt, nickel and palladium, and an acidic oxide support consisting essentially of a substantially alkali metal-free mixture of about 5 wt % to 60 wt % of a crystalline zeolite, such as zeolite Y, and about 95 wt % to 40 wt % of a silica-alumina cracking catalyst. The dual function catalyst is produced by impregnating the support with a solution of the desired hydrogenation metal(s) followed by calcining in an oxidizing atmosphere to convert the hydrogenating component to the oxide form. The catalyst is then reduced, by contact with hydrogen for, e.g., 4.0 hours at 900° F. (482° C.). The resultant catalyst is shown to exhibit benzene conversions of 26.3% to 35.1% at a reaction temperature of 174-183° C. and a hydrogen pressure of 500 psig.
A further process is described in U.S. Pat. No. 6,037,513 (hereinafter the '513 Patent), in which an aromatic hydrocarbon, such as benzene, is contacted with hydrogen in the presence of a bifunctional catalyst which has both hydrogenation activity and alkylation activity. In particular, the catalyst comprises a metal having hydrogenation activity, such as palladium, and a crystalline inorganic oxide material having alkylation activity and an X-ray diffraction pattern including d-spacing maxima at 12.4±0.25, 6.9±0.15, 3.57±0.07, and 3.42±0.07 Angstrom. The crystalline inorganic oxide material may be composited with a binder or matrix. The catalyst is produced by impregnating the crystalline inorganic oxide material with an aqueous solution of a palladium salt and, then prior to employing the catalyst in a hydroalkylation reaction, treating the catalyst with 50 cc/min of flowing hydrogen for 2.0 hours at 300° C. and 1 atm pressure. Although not stated in the '513 Patent, this hydrogen treatment is employed to activate the catalyst by converting the palladium salt to a more active form of palladium.
Experience operating the process described in the '513 Patent has shown that the activated catalyst can convert about 34 wt % benzene in a single-pass through a fixed bed reactor nominally operating at 145° C., 165 psig (1138 kPa gauge) total pressure, 2.5 weight benzene/weight catalyst/hour weight-hourly space velocity (WHSV) with an H2/benzene feed molar ratio of 0.7. Thus, at least 60 wt % of the benzene remains unconverted after each pass of the benzene feed through the hydroalkylation reactor. There is significant interest in increasing the benzene conversion of the catalyst provided this can be achieved without reducing its cyclohexylbenzene selectivity.
According to the present invention, it has now been found that the aromatic conversion activity of hydroalkylation catalysts, such as those disclosed in the '513 Patent, can be improved, desirably without reduction in cycloalkylaromatic selectivity by subjecting the catalyst to a second hydrogen activation treatment after the catalyst is exposed to a hydroalkylation condition between the first and second hydrogen treatments.
In this respect, it is known that catalysts, particularly supported metal catalysts, are sometimes exposed to hydrogen treatments after being on stream in a catalytic reaction in order to restore their lost activity. Such treatments are often referred to in the art of catalysis as rejuvenation. Such treatments, however, at best restore catalytic activity, but do not increase it above the start of run activity levels. In fact, while such treatments can restore some of the lost activity, they often cannot restore the full start of run activity. Surprisingly, the currently disclosed treatment results in a different outcome, namely, the catalyst treated by the three-step activation process of the current disclosure has higher activity than can be achieved by the prior-art one-step activation process.